The "4th Dimension" is a modification for the expansion Forged Alliance for Supreme Commander which adds new content like completely new designed units, explosions and other effects. But this mod does not only add new content, it also changes parts of the game like balancing and gameplay.
The goal of this mod is to make SC feel more realistic (as long as it is better for the gameplay) and to improve the balancing and optics of the game. For example nearly all muzzle velocities have been increased, and many units move much slower now.
Another big aspect of this mod is the fact, that in 4th Dimension nearly all units have their right to exist, even in later tech levels, while in Supreme Commander you only see very few low tech units on the battlefield.
Beside a new veteran system for vanilla Supreme Commander, which lets stronger units count more than weaker units and which gives units bigger buffs, the mod changes also the tarmac sizes and explosion effects.